X-ray apparatus with dual monochromators

ABSTRACT

X-ray apparatus, consisting of a single X-ray tube which is adapted to generate X-rays and a first optic which is adapted to focus a first portion of the X-rays onto a region of a sample via a first beam path, thereby generating first scattered X-rays from the region. The apparatus also includes a second optic which is adapted to focus a second portion of the X-rays onto the region of the sample via a second beam path, different from the first beam path, thereby generating second scattered X-rays from the region. A detector assembly simultaneously collects the first and the second scattered X-rays.

FIELD OF THE INVENTION

The present invention relates generally to analytic instruments, andparticularly to analytic instruments and methods using X-ray scattering.

BACKGROUND OF THE INVENTION

X-ray reflectometry (XRR) is a well-known technique for measuring thethickness, density and surface quality of thin film layers deposited ona substrate. Such reflectometers typically operate by irradiating asample with a beam of X-rays at grazing incidence, i.e., at a smallangle relative to the surface of the sample, in the vicinity of thetotal external reflection angle of the sample material. Measurement ofthe X-ray intensity reflected from the sample as a function of anglegives a pattern of interference fringes, which is analyzed to determinethe properties of the film layers responsible for creating the fringepattern. The X-ray intensity measurements are commonly made using aposition-sensitive detector.

Various types of position-sensitive X-ray detectors are known in the artof reflectometry. Solid-state arrays typically comprise multipledetector elements, which are read out by a charge-coupled device (CCD)or other scanning mechanism. The signals at low angles, below the totalexternal reflection angle, are usually much stronger than the signalsabove this angle, although in both cases there may be random, relativelylarge signal excursions.

In order to obtain accurate measurements of reflected beams, it isnecessary to precisely calibrate the angular scale of the reflection.Such a calibration requires, inter alia, exact control of the zero angleof reflection, so that the angle of the reflected beam relative to thesurface can be determined accurately. (In the context of the presentpatent application and in the claims, the term “zero angle” refers tothe orientation of a tangent to the reflecting surface at the point ofincidence of the radiation.) To make reflectometric measurements withoptimal accuracy, the zero angle at the measurement point should beknown to within 0.001°.

Although semiconductor wafers appear to be flat, in practice waferstypically deform slightly when held by a vacuum chuck during productionor inspection. The deformation is due both to the vacuum force exertedby the chuck and to the weight of the wafer itself. Furthermore, thechuck may have imperfections, such as a slight bend in its axis, thatcause deviations in the zero angle of the wafer as it rotates. As aresult, inclination of the surface at different sample points on thesurface of a wafer may vary by as much as 0.1–0.2°. Therefore, toperform accurate reflectometric measurements at a well-definedmeasurement point, it becomes necessary to recalibrate the zero angle ateach point that is tested on the wafer surface.

In an X-ray reflectometer, the irradiating X-ray beam is typicallyfiltered in a monochromator, and is also focused onto a small region ofthe surface being analyzed. A number of systems which act both as amonochromator and as a focusing element are known in the art. Suchcombined systems typically use curved crystals, the operation of whichis based on the Bragg X-ray reflection law:nλ=2d sin(θ_(B))  (1)

where n is a diffractive order of X-rays of wavelength λ diffracted fromcrystal planes having a spacing d, and θ_(B) is the angle between anincident X-ray beam and the crystal planes.

U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure isincorporated herein by reference, describes an X-ray spectrometer basedon a curved crystal monochromator. The monochromator has the shape of atapered logarithmic spiral, which is described as achieving a finerfocal spot on a sample surface than prior art monochromators, a numberof which are also described in the disclosure.

U.S. Pat. No. 6,711,232, to Janik, whose disclosure is incorporatedherein by reference, describes X-ray measurements using a linear X-raysource having an axis at right angles to the surface of a sample beingmeasured. A beam from the source is focused by a reflector onto thesample. Alternative arrangements describe two linear sources with axesat right angles to the sample surface. The two sources generaterespective X-ray beams which are focused by two reflectors onto thesample.

XOS Inc., of Albany, N.Y., produce the Doubly-Bent Focusing CrystalOptic, which comprises a single crystal having two orthogonal radii ofcurvatures. The two radii of curvatures enable the crystal to act bothas a focusing element and as a monochromator. Crystals having two radiiof curvatures in different directions, such as those exemplified by theDoubly-Bent Focusing Crystal Optic, are herein termed doubly-curvedcrystals (DCCs) or DCC optics. DCC optics are formed from crystals suchas mica, quartz, or silicon.

DCC optics typically incorporate an idea first suggested in 1882 byRowland for optical gratings. Rowland demonstrated that a grating ruledon a spherical mirror having a radius 2R would focus all orders ofspectra from the mirror onto a circle of radius R if the source ofradiation irradiating the curved grating is also on the circle. Thecircle is termed the Rowland circle. The X-ray source, the DCC optic,and the focused image of the X-ray source all lie on the Rowland circle.

SUMMARY OF THE INVENTION

In embodiments of the present invention a plurality of focusing optics,typically two optics, focus a respective plurality of irradiating X-raybeams from different directions onto a single region, such as a regionof a semiconductor wafer surface being inspected by the X-rays.Typically, the focusing optics comprise crystal monochromators, whichboth focus and monochromatize the X-ray beam. Each of the beams scattersfrom the single region, and the scattered beams are collected by adetector assembly, which may comprise a respective plurality ofdetectors. Focusing the plurality of beams onto the single region, andthen collecting the scattered beams, enables multiple simultaneous,independent X-ray scattering measurements to be made of the singleregion.

The multiple independent measurements, and the increased irradiatingpower, substantially increase an overall signal-to-noise ratio (SNR) forthe system. In addition to improving the overall SNR, using a pluralityof irradiating X-ray beams following different beam paths enables randomlarge signal excursions from one of the beams to be rejected.Furthermore, in the case of wafer inspection, the different incomingbeam directions of the different paths facilitate accurate determinationof the zero angle at the irradiated region.

The X-ray beams are generated from a single anode in a single X-raytube, the single anode effectively acting as an X-ray point source.Using a single X-ray tube to produce multiple X-ray beams reduces boththe cost and the size of the apparatus, compared to prior artmultiple-beam systems.

In an exemplary embodiment, there are two crystal monochromators whichare each formed as doubly curved crystals (DCCs), each of the crystalsbeing curved to have substantially similar radii of curvature. Thegeometry of the paths of the beams reflected by the DCCs enables thelatter to be arranged on a circle orthogonal to a line between the X-raysource and the single region, the circle being centered on the line.Such an arrangement may be implemented without complicated alignment.

Although the embodiments described herein are directed primarily to XRRmeasurements, the principles of the present invention may also beapplied in other types of X-ray scattering measurements, such as X-raydiffraction (XRD), as well as other radiation-based systems for analysisand/or investigation of materials and/or thin film measurements.

There is therefore provided, according to an embodiment of the presentinvention, an X-ray apparatus, including:

a single X-ray tube which is adapted to generate X-rays;

a first optic which is adapted to focus a first portion of the X-raysonto a region of a sample via a first beam path, thereby generatingfirst scattered X-rays from the region;

a second optic which is adapted to focus a second portion of the X-raysonto the region of the sample via a second beam path, different from thefirst beam path, thereby generating second scattered X-rays from theregion; and

a detector assembly which is adapted to simultaneously collect the firstand second scattered X-rays.

In an embodiment, at least one of the first optic and the second opticis adapted to act as a monochromator.

In an alternative embodiment, the first and the second optics includedoubly-curved crystals having identical curvatures. Typically, the firstoptic and the second optic are symmetrically disposed about a linejoining the X-ray tube and the region.

In a disclosed embodiment, the sample defines a plane, and the sampleand the first and the second beam paths are configured so that firstincident beam elevation angles between the first portion of the X-raysand the plane are equal to second incident beam elevation angles betweenthe second portion of the X-rays and the plane. The first and secondincident beam elevation angles may include angles between zero and atotal external reflection angle of the sample. Alternatively oradditionally, the first and second incident beam elevation angles mayinclude angles greater than a total external reflection angle of thesample.

In one embodiment, the detector assembly includes a first detector arraywhich collects the first scattered X-rays and a second detector arraywhich collects the second scattered X-rays. Typically, the detectorassembly generates respective first and second signals responsively tothe first and the second scattered X-rays, and the apparatus furtherincludes a processor which is adapted to combine the first and secondsignals to output a spectrum, and to determine a property of the samplein response to the spectrum. The property may include a tilt angle ofthe sample.

The scattered X-rays may include reflected X-rays, and the spectrum mayinclude an X-ray reflectance spectrum. The sample may include a thinsurface layer, and the processor may be adapted to analyze thereflectance spectrum so as to determine at least one of a thickness, adensity and a surface roughness of the thin surface layer.

Alternatively, the scattered X-rays include diffracted X-rays, and thespectrum includes an X-ray diffraction spectrum.

Typically, the sample includes a semiconductor wafer, and the apparatusis adapted to perform X-ray reflectometry on the semiconductor wafer,and the apparatus further includes a processor which receives an outputof the detector assembly produced in response to collection of the firstand second scattered X-rays therein, and which determines a property ofa layer in the semiconductor wafer in response to the output.

In some embodiments, the apparatus includes one or more third opticswhich are adapted to focus respective one or more third portions of theX-rays onto the region of the sample via respective one or more thirdbeam paths, thereby generating respective one or more third scatteredX-rays from the region, each of the first, second, and the one or morethird beam paths being different, and the detector assembly is adaptedto simultaneously collect the one or more third scattered X-rays.

Typically, the single X-ray tube is operative as an approximate pointsource.

In another disclosed embodiment, the single X-ray tube, the first optic,and the region define a first Rowland circle, and the single X-ray tube,the second optic, and the region define a second Rowland circle, and thefirst and the second Rowland circles have equal radii.

In a further disclosed embodiment, the single X-ray tube, the firstoptic, the second optic, and the region lie in a single plane.

There is further provided, according to an embodiment of the presentinvention, a method for investigating a sample, including:

generating X-rays in a single X-ray tube;

directing and focusing with a first optic a first portion of the X-raysonto a region of the sample via a first beam path, thereby generatingfirst scattered X-rays from the region;

directing and focusing with a second optic a second portion of theX-rays onto the region of the sample via a second beam path, differentfrom the first beam path, thereby generating second scattered X-raysfrom the region; and

simultaneously collecting the first and the second scattered X-rays.

Typically, at least one of the first optic and the second optic isadapted to act as a monochromator, and the first and the second opticsinclude doubly-curved crystals having identical curvatures. Typically,the method includes symmetrically disposing the first optic and thesecond optic about a line joining the single X-ray tube and the region.

In a disclosed embodiment, the sample defines a plane, and the sampleand the first and the second beam paths are configured so that firstincident beam elevation angles between the first portion of the X-raysand the plane are equal to second incident beam angles between thesecond portion of the X-rays and the plane.

The first and second incident beam elevation angles may include anglesbetween zero and a total external reflection angle of the sample.Alternatively or additionally, the first and second incident beamelevation angles include angles greater than a total external reflectionangle of the sample.

In one embodiment, collecting the first and the second scattered X-raysincludes collecting the first and the second scattered X-rays in adetector assembly including a first detector array which collects thefirst scattered X-rays and a second detector array which collects thesecond scattered X-rays. Typically, the detector assembly generatesrespective first and second signals from the first and the secondscattered X-rays, and the method includes combining the first and secondsignals to output a spectrum, and determining a property of the samplein response to the spectrum. The property may include a tilt angle ofthe sample.

The scattered X-rays may include reflected X-rays, and the spectrumincludes an X-ray reflectance spectrum.

The sample may include a thin surface layer, and the method typicallyincludes analyzing the reflectance spectrum so as to determine at leastone of a thickness, a density and a surface roughness of the thinsurface layer.

Alternatively or additionally, the scattered X-rays include diffractedX-rays, and the spectrum includes an X-ray diffraction spectrum.

In a disclosed embodiment, the sample includes a semiconductor wafer.Typically, investigating the sample includes performing X-rayreflectometry on the semiconductor wafer, and the method furtherincludes generating an output from a detector assembly in response tocollecting the first and the second scattered X-rays therein, andprocessing the output to determine a property of a layer included in thesemiconductor wafer.

In another embodiment, the method includes directing and focusing withone or more third optics respective one or more third portions of theX-rays onto the region of the sample via respective one or more thirdbeam paths, thereby generating respective one or more third scatteredX-rays from the region, and simultaneously collecting the first and thesecond and the one or more third scattered X-rays, and wherein each ofthe first, second, and the one or more third beam paths are configuredto be different.

In yet another disclosed embodiment, the single X-ray tube is operativeas an approximate point source.

In an alternative disclosed embodiment, the single X-ray tube, the firstoptic, and the region define a first Rowland circle, and the singleX-ray tube, the second optic, and the region define a second Rowlandcircle, and the first and the second Rowland circles have equal radii.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings, a brief description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B illustrate schematic top and side views of an X-rayapparatus, according to an embodiment of the present invention; and

FIG. 2 shows schematic simulated plots of distributions generated by theapparatus of FIGS. 1A and 1B, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to FIG. 1A and FIG. 1B, which are schematic topand side views of an X-ray apparatus 10, according to an embodiment ofthe present invention. Like numerals in FIGS. 1A and 1B identify likeelements of the apparatus. The description herein is directed, by way ofexample, to X-ray examination, typically comprising X-ray reflectometry(XRR) and/or diffractometry (XRD), of a region on a surface of asemiconductor wafer. The examination is typically to determine aproperty, such as a thickness, a density and/or a surface roughness, ofa thin surface layer of the wafer. It will be understood, however, thatthe principles of the present invention may be applied to irradiation ofsubstantially any type of region to which the irradiating X-rays can bedirected. Such regions include the interior and/or the surface of gases,liquids and colloids, as well as the interior and/or the surface ofsolids.

A single X-ray tube 11, such as an XTF 5011 produced by OxfordInstruments of Scotts Valley, Calif., generates X-rays from a singleanode 12. Anode 12 acts as an approximate point source S by the X-raytube focusing its electrons onto the anode in a circle of the order of50 microns diameter. Typically, the X-rays generated are soft X-rays,for example having energies of about 8.05 keV (CuKa₁) or 5.4 keV(CrKa₁). The X-rays are confined, by two apertures 13, 15 in a shield14, into two separate beams 30 and 36 which each diverge from anode 12.Beams 30 and 36 have generally similar angles of divergence, typicallyapproximately equal to 5°.

Diverging beam 30 irradiates a first X-ray optic 16, and beam 36irradiates a second X-ray optic 18. Each optic acts as both a focusingelement and as a monochromator. Hereinbelow, by way of example, optics16 and 18 are assumed to comprise substantially similar doubly curvedcrystal (DCC) optics, and are also referred to as DCC 16 and DCC 18.

An article titled “Doubly Curved Crystals Direct X-rays” by Chen et al.,published in the August 2003 edition of OE magazine, which may be foundat oemagazine.com/fromTheMagazine/aug03/curvedcrystals.html, isincorporated herein by reference. The article describes how a DCC optichaving two different radii of curvatures in orthogonal directionsoperates. A first radius of curvature of the DCC optic is set at:r _(H)=2R,  (2)

where R is the radius of the Rowland circle.

The DCC optic also has a second radius of curvature, orthogonal to thefirst radius of curvature, having a value given by:r _(V)=2R sin²(θ_(B))  (3)

where θ_(B) is the angle between an incident X-ray beam and the planesof the DCC optic, and may be found from equation (1) in the Backgroundof the Invention.

DCCs 16 and 18 have orthogonal radii r_(H) and r_(V), as given byequations (2) and (3) above.

DCC 16 focuses beam 30 into a converging monochromatic beam 32 whichconverges to a region 22, also referred to herein as region R, on asurface 21 of a wafer 20. Wafer 20 is typically held on a mountingassembly, such as a motion stage 23, so that surface 21 is substantiallyhorizontal, the mounting assembly allowing accurate adjustment of theposition and the orientation of the wafer in all three dimensions, wherean X-Y plane is defined by surface 21. Anode 12, DCC 16, and region 22define a first plane, and they also define a first Rowland circle, ofradius R, in the plane.

Incident X-rays from beam 32 scatter at region 22 to form a scatteredbeam 34, which is collected by a detector sub-assembly 26, comprising adetector array 27, such as a charge-coupled device (CCD). A detectorassembly which may be advantageously used to perform the function ofsub-assembly 26 is described in more detail in U.S. Pat. No. 6,512,814,which is incorporated herein by reference. In the specification and inthe claims, the term “scatter,” as well as derived terms such as“scattered” and “scattering,” is assumed to comprise any sort ofemission from a sample that is induced in response to an incidentradiation beam, including reflection and/or diffraction of the incidentradiation beam. Thus, scattered beam 34 may comprise reflection of beam32 from region 22, and/or diffraction of beam 32 from the region.

In a similar manner to that of DCC 16, DCC 18 focuses beam 36 into aconverging monochromatic beam 38 which converges to region 22. Anode 12,DCC 18, and region 22 define a second plane, and as described above forthe first Rowland circle, they also define a second Rowland circle.Incident X-rays from beam 38 scatter at region 22 to form a scatteredbeam 40, which is collected by a detector sub-assembly 24, comprising adetector array 25. Sub-assembly 24 is typically generally similar tosub-assembly 26, and both sub-assemblies collect their beamssimultaneously. In one embodiment of the present invention, the twosub-assemblies are configured as a detector assembly 29.

Since DCC 16 and DCC 18 are substantially similar in composition andconstruction, the wavelength of beams 32 and 38 are substantially thesame. In an embodiment of the present invention, DCC 16 and DCC 18 arepositioned so that beams 32 and 38 both make substantially similarelevation angles φ with surface 20 and so that the first and the secondplanes referred to above are substantially coincident. The elevationangle for each beam 32 and 38 may typically be in a range of 0–5° forXRR, or in a range of 30–40° for XRD. Appropriate elevation angle rangesfor other types of X-ray irradiation will be apparent to those skilledin the art.

In a disclosed embodiment the positions of crystal 16 and crystal 18 areadjusted so that beams 30 and 36 make an angle θ with each other ofapproximately 28°, although the DCCs may be positioned to formsubstantially any convenient value of θ. Typically, DCC 16 and DCC 18are substantially equidistant from a line SR connecting anode 12 (S) andregion 22 (R), so that the first and the second Rowland circles havesubstantially the same radius.

A signal processor 60 receives and analyzes the output of each detectorsub-assembly 24, 26 so as to determine respective distributions 62, 64of the flux of X-ray photons scattered from region 22 as a function ofelevation angle at a given X-ray energy. By way of example, distribution62 is illustrated schematically in FIG. 1B, and in more detail in FIG.2. Typically, wafer 20 has one or more thin surface layers, such as thinfilms, at region 22. Consequently, distributions 62 and 64 exhibit astructure that is characteristic of interference and/or diffractioneffects due to the surface layer and interfaces between the layers.

While the paths followed by beams 32 and 38 are different, both makesimilar small elevation angles with surface 20, and since the beams havesimilar wavelengths, distributions 62 and 64 are expected to besubstantially the same. The expected similarity of the distributions maybe used by processor 60 to significantly improve measurements of thedistributions and derived measurements thereof, as exemplified belowwith reference to FIG. 2.

As noted earlier, stage 23 shifts wafer 22 in the X-Y plane to enableapparatus 10 to measure spectra at multiple locations on the surface ofthe wafer. The surface tilt angle of the wafer (i.e., the angle ofdeviation between a plane that is locally tangent to the surface and thereference X-Y plane) on stage 23 may not be perfectly uniform over theentire surface of the wafer. In a typical use of the apparatus as areflectometer, wafer 20 is a reference wafer which is held in place onstage 23 by suction exerted through vacuum ports (not shown) in thesurface of the stage. Under these circumstances, the reference waferconforms to the shape of the stage, with deformations due to the forceof the suction. As a result, the local tilt angle of the wafer may varyfrom point to point on the wafer surface. Accurate XRR measurement,however, requires that the tilt angle at each point be known and takeninto account, so that apparatus 10 is used to generate a tilt map of thetilt angle variations over the reference wafer, and these variations arethen used when wafer 20 is a production wafer.

Techniques described in U.S. patent application Ser. Nos. 10/313,280,10/364,883, and 10/689,314, or in U.S. Pat. No. 6,680,996, which areassigned to the assignee of the present invention and which areincorporated herein by reference, may be used to generate the tilt map.

Since apparatus 10 uses simultaneous measurements of beams 32 and 38,the time required to prepare the tilt map is substantially reducedrelative to prior art single-beam systems. The tilt angles, andinterpolated tilt angles derived therefrom, may then be applied in orderto correct the XRR results for wafer 20 as a production wafer.Typically, the angular scale of each distribution 62 and 64 of aproduction wafer is adjusted to account for the local tilt at the pointat which the distribution was measured. Alternatively, the tilt angle ofstage 23 or the positions of X-ray source S and detector sub-assemblies24 and 26 may be adjusted to compensate for the local tilt.

FIG. 2 shows schematic simulated plots of distributions 62 and 64,according to an embodiment of the present invention. The plots show, ona logarithmic scale, the number of counts n(j) accumulated at each pixelof array 25 and array 27 as a function of reflection angle. Thedistributions are assumed to be generated after allowances for tilt ofregion 22 have been incorporated in the plots. Typically, themeasurements of tilt are made as described above. For clarity, thecounts scale of distribution 62 is different from that of distribution64 so that the two distributions are clearly seen. Since bothdistributions 62 and 64 are determined for the same region 22 usingX-rays of substantially the same wavelength, and since corrections fortilt of region 22 have been incorporated in the plots, the distributionsare expected to be substantially identical.

In practice, however, random noise causes the two distributions to bedifferent. In apparatus 10 the signal to noise ratio (SNR) is increased,compared to the SNR of single beam reflectometers or diffractometers, byaveraging the two distributions in processor 60 to output a finalspectrum. Furthermore, in apparatus 10, random, relatively large signalexcursions, such as that exemplified by a peak 66, may be recognized andsubtracted out by processor 60 before outputting the final spectrum. Forexample, the processor may fit distributions 62 and 64 to a curve basedon the averaged distributions, and subtract out results that are greaterthan a predetermined number of standard deviations, such as three, fromthe fitted curve. After removing outliers in this fashion, the processortypically repeats the averaging and fitting process.

It will be appreciated that while the embodiments above relate to X-rayapparatus that has two separate beams, the scope of the presentinvention includes X-ray apparatus having three or more separate beams.As in apparatus 10, the respective focusing elements for each of thebeams of such a multiple beam apparatus are typically approximatelyequidistant from line SR (FIG. 1A). It will also be appreciated thatembodiments of the present invention may be advantageously used as partof a cluster tool, and/or in situ in a processing chamber.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. An X-ray apparatus, comprising: a single X-ray tube which is adaptedto generate X-rays: a first optic which is adapted to focus a firstportion of the X-rays onto a region of a sample via a first beam path,thereby generating first scattered X-rays from the region; a secondoptic which is adapted to focus a second portion of the X-rays onto theregion of the sample via a second beam path, different from the firstbeam path, thereby generating second scattered X-rays from the region;and a detector assembly which is adapted to simultaneously collect thefirst and second scattered X-rays, wherein the detector assemblycomprises a first detector array which collects the first scatteredX-rays and a second detector array which collects the second scatteredX-rays.
 2. The apparatus according to claim 1, where at least one of thefirst optic and the second optic is adapted to act as a monochromator.3. The apparatus according to claim 1, wherein the first and the secondoptics comprise doubly-curved crystals having identical curvatures. 4.The apparatus according to claim 1, wherein the first optic and thesecond optic are symmetrically disposed about a line joining the X-raytube and the region.
 5. The apparatus according to claim 1, wherein thefirst detector array and the second detector array generate respectivefirst and second signals responsively to the first and the secondscattered X-rays, and comprising a processor which is adapted to combinethe first and second signals to output a spectrum, and to determine aproperty of the sample in response to the spectrum.
 6. The apparatusaccording to claim 5, wherein the property comprises a tilt angle of thesample.
 7. The apparatus according to claim 5, wherein the first andsecond scattered X-rays comprise reflected X-rays, and wherein thespectrum comprises an X-ray reflectance spectrum.
 8. The apparatusaccording to claim 7, wherein the sample comprises a thin surface layer,and wherein the processor is adapted to analyze the reflectance spectrumso as to determine at least one of a thickness, a density and a surfaceroughness of the thin surface layer.
 9. The apparatus according to claim5, wherein the first and second scattered X-rays comprise diffractedX-rays, and wherein the spectrum comprises an X-ray diffractionspectrum.
 10. The apparatus according to claim 1, wherein the samplecomprises a semiconductor wafer.
 11. The apparatus according to claim10, wherein the apparatus is adapted to perform X-ray reflectometry onthe semiconductor wafer, and further comprising a processor whichreceives an output of detector assembly produced in response tocollection of the first and second scattered X-rays therein, and whichdetermines a property of a layer of the semiconductor wafer in responseto the output.
 12. The apparatus according to claim 1, and comprisingone or more third optics which are adapted to focus respective one ormore third portions of the X-rays onto the region of the sample viarespective one or more third beam paths, thereby generating respectiveone or more third scattered X-rays from the region, each of the first,second, and the one or more third beam paths being different, andwherein the detector assembly is adapted to simultaneously collect theone or more third scattered X-rays.
 13. The apparatus according to claim1, wherein the single X-ray tube is operative as an approximate pointsource.
 14. The apparatus according to claim 1, wherein the single X-raytube, the first optic, and the region define a first Rowland circle, andwherein the single X-ray tube, the second optic, and the region define asecond Rowland circle, and wherein the first and the second Rowlandcircles have equal radii.
 15. The apparatus according to claim 1,wherein the single X-ray tube, the first optic, the second optic, andthe region lie in a single plane.
 16. An X-ray apparatus, comprising: asingle X-ray tube which is adapted to generate X-rays: a first opticwhich is adapted to focus a first portion of the X-rays onto a region ofa sample via a first beam path, thereby generating first scatteredX-rays from the region; a second optic which is adapted to focus asecond portion of the X-rays onto the region of the sample via a secondbeam path, different from the first beam path, thereby generating secondscattered X-rays from the region; and a detector assembly which isadapted to simultaneously collect the first and second scattered X-rays,wherein the sample defines a plane, and wherein the sample and the firstand the second beam paths are configured so that first incident beamelevation angles between the first portion of the X-rays and the planeare equal to second incident beam elevation angles between the secondportion of the X-rays and the plane.
 17. The apparatus according toclaim 16, wherein the first and second incident beam elevation anglescomprise angles between zero and a total external reflection angle ofthe sample.
 18. The apparatus according to claim 16, wherein the firstand second incident beam elevation angles comprise angles greater than atotal external reflection angle of the sample.
 19. A method forinvestigating a sample, comprising: generating X-rays in a single X-raytube; directing and focusing with a first optic a first portion of theX-rays onto a region of the sample via a first beam path, therebygenerating first scattered X-rays from the region; directing andfocusing with a second optic a second portion of the X-rays onto theregion of the sample via a second beam path, different from the firstbeam path, thereby generating second scattered X-rays from the region;and simultaneously collecting the first and second scattered X-rays,wherein collecting the first and the second scattered X-rays comprisescollecting the first and the second scattered X-rays in a detectorassembly comprising a first detector array which collects the firstscattered X-rays and a second detector array which collects the secondscattered X-rays.
 20. The method according to claim 19, wherein at leastone of the first optic and the second optic is adapted to act as amonochromator.
 21. The method according to claim 19, wherein the firstand the second optics comprise doubly-curved crystals having identicalcurvatures.
 22. The method according to claim 19, wherein the firstoptic and the second optic are symmetrically disposed about a linejoining the single X-ray tube and the region.
 23. The method accordingto claim 19, wherein the detector assembly generates respective firstand second signals from the first and the second scattered X-rays, andcomprising combining the first and second signals to output a spectrum,and determining a property of the sample in response to the spectrum.24. The method according to claim 23, wherein the property comprises atilt angle of the sample.
 25. The method according to claim 23, whereinthe first and second scattered X-rays comprise reflected X-rays, andwherein the spectrum comprises an X-ray reflectance spectrum.
 26. Themethod according to claim 25, wherein the sample comprises a thinsurface layer, and comprising analyzing the reflectance spectrum so asto determine at least one of a thickness, a density and a surfaceroughness of the thin surface layer.
 27. The method according to claim23, wherein the first and second scattered X-rays comprise diffractedX-rays, and wherein the spectrum comprises an X-ray diffractionspectrum.
 28. The method according to claim 19, wherein the samplecomprises a semiconductor wafer.
 29. The method according to claim 28,wherein investigating the sample comprises performing X-rayreflectometry on the semiconductor wafer, and comprising generating anoutput from a detector assembly in response to collecting the first andthe second scattered X-rays therein, and processing the output todetermine a property of a layer comprised in the semiconductor wafer.30. The method according; to claim 19, and comprising directing andfocusing with one or more third respective one or more third portions ofthe X-rays onto the region of the sample via respective one or morethird beam paths, thereby generating respective one or more thirdscattered X-rays from the region, and simultaneously collecting thefirst and the second and the one or more third scattered X-rays, andwherein each of the first, second, and the one or more third beam pathsare configured to be different.
 31. The method according; to claim 19,wherein the single X-ray tube is operative as an approximate pointsource.
 32. The method according; to claim 19, wherein the single X-raytube, the first optic, and the region define a first Rowland circle, andwherein the single X-ray tube, the second optic, and the region define asecond Rowland circle, and wherein the first and the second Rowlandcircles have equal radii.
 33. The method according; to claim 19, whereinthe single X-ray tube, the first optic, the second optic, and the regionlie in a single plane.
 34. A method for investigating a sample,comprising: generating X-rays in a single X-ray tube; directing andfocusing with a first optic a first portion of the X-rays onto a regionof the sample via a first beam path, thereby generating first scatteredX-rays from the region; directing and focusing with a second optic asecond portion of the X-rays onto the region of the sample via a secondbeam path, different from the first beam path, thereby generating secondscattered X-rays from the region; and simultaneously collecting thefirst and second scattered X-rays, wherein the sample defines a plane,and wherein the sample and the first and the second beam paths areconfigured so that first incident beam elevation angles between thefirst portion of the X-rays and the plane are equal to second incidentbeam elevation angles between the second portion of the X-rays and theplane.
 35. The method according to claim 34, wherein the first andsecond incident beam elevation angles comprise angles between zero and atotal external reflection angle of the sample.
 36. The method accordingto claim 34, wherein the first and second incident beam elevation anglescomprise angles greater than a total external reflection angle of thesample.